Optical fiber of the type used in communications is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney, et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc., has been found most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, precursor, reactant-containing gases, such as SiCl and GeCl , are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a preform tube. Once a sufficient number of layers have been deposited, the preform tube is then heated to cause it to be collapsed to yield a preform or preform rod as it is often called.
Increased demand for optical fiber has prompted efforts to increase the productivity of the MCVD process. However, the MCVD process rate is limited by the thickness of the wall of the substrate tube. To obtain optical fiber having optimal geometrical and optical characteristics, the preform must have a core-to-cladding mass ratio within specified limits. Increasing the mass of the substrate tube to obtain a larger preform requires that the wall of the substrate tube be made thicker. However, increasing the thickness of the wall of the substrate tube reduces the rate of heat transfer to the reactant-containing gases, thereby increasing the time required to deposit each layer of glass particulates. If the wall of the substrate tube is too thick, then insufficient heat transfer may occur, which may result in the formation of bubbles or incomplete sintering.
One way in which the productivity of the MCVD process can be increased is first to produce an undercladded preform, having a larger than desired core-to-cladding mass ratio. This preform is inserted into a glass tube which is referred to as an overcladding tube and which is then collapsed onto the preform. This is referred to as the rod and tube technique. It is desirable that any added eccentricity of material about the preform core due to overcladding should be minimized.
If the undercladding is not substantially straight, difficulties are encountered when the rod is inserted into the tube. This also may lead to problems when the tube is collapsed on the rod. Contact of the preform with the inside surface of the tube has not been found to be detrimental for present proof test levels of interest. However, radial misalignment between the overcladding tube and the undercladded preform should be minimized, otherwise the resultant drawn fiber core may be too eccentric which inhibits proper splicing of the drawn fiber to another.
Optical fiber preform tube straightening is not new. For example, in U.S. Pat. No. 4,477,273, methods and apparatus are used for straightening and configuring an optical preform tube. A graphite roller which is mounted on a carriage that supports a torch is moved manually into engagement with the preform tube at one end thereof and then moved along the tube with the torch during a collapse mode. However, the manual movement of a roller into engagement with a preform rod may be excessive and it may become embedded in the rod to such an extent that as it is moved with the torch it could be urged against a mass of the preform rod and cause damage thereto.
What is needed and what does not appear to be available in the prior art are automatic methods and apparatus for causing a preform rod to be substantially straight so that it is suitable for trouble-free insertion into a tube in a rod and tube process. The sought-after methods and apparatus desirably should be adaptable to existing apparatus and be controllable for a variety of conditions.